For the measuring, or registering, of a process variable of a media flowing in pipelines, especially for the registering of flow-dynamic and/or rheological, measured variables of fluids, inline measuring devices working according to the most varied of physical principles are used in measurements and automation technology. For registering the particular process variable, for example a mass flow, a density and/or a viscosity of a fluid, the inline measuring device has a corresponding, most often physical-to-electrical, measurement pickup, or transducer, which is inserted into the course of the line conveying the medium and which serves for producing at least one measurement signal, especially an electrical measurement signal, representing, as accurately as possible, the primarily registered, process variable. The measurement pickup is, in such case, connected with the pipeline e.g. by means of flanges, tightly against leakage of the medium, especially pressure-tightly, and, mostly, also lastingly.
For operating the measurement pickup, especially also for the further processing or evaluating of the at least one measurement signal, such is additionally attached to a corresponding measuring device electronics. In the case of inline measuring devices of the described type, the measuring device electronics is, in turn, usually connected via an attached data transmission system, with other inline measuring devices and/or with appropriate process control computers, to which they transmit the measured signals e.g. via (4 mA to 20 mA)-current loops and/or digital data bus. Serving often, in such case, for data transmission systems are, especially serial, fieldbus systems, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, together with the corresponding transmission protocols.
By means of the process control computer, the transmitted, measured-value signals can be processed further and visualized e.g. on monitors as corresponding measurement results and/or they can be converted into control signals for process-influencing actuators, such as e.g. solenoidal valves, electric motors, etc. For the accommodating of the measuring device electronics, such inline measuring devices include further an electronics housing, which, as proposed e.g. in WO-A 00/36 379, can be arranged remotely from the measurement pickup and connected with such only over a flexible line, or which, as shown e.g. also in EP-A 1 296 128 or WO-A 02/099363, is arranged directly on the measurement pickup, especially in the form of a measurement pickup housing, which houses the measurement pickup.
For the measuring of, especially, mass flows, e.g. flow rates, densities and/or viscosities of flowing media, inline measuring devices having a vibration-type measurement pickup for insertion into the course of a pipeline conveying the fluid to be measured have become established over a considerable period of time. Such inline measuring devices, or measurement pickups, their mechanical construction or also measuring and evaluation processes producing corresponding measurement signals are described e.g. in EP-A 189 230, EP-A 527 176, EP-A 1 154 243, EP-A 1 158 289, EP-A 1 223 412, EP-A 1 296 128, U.S. Pat. Nos. 4,524,610, 4,768,384, 4,801,897, 4,823,614, 5,231,884, 5,359,881, 5,602,345, 5,661,232, 5,687,100, 6,006,609, 6,327,915, 6,343,517, 6,354,154, 6,487,917, 6,513,393, 6,634,241, US-A 2003/0154804, US-A 2003/0097881, US-A 2003/0097884, WO-A 88 02 476, WO-A 95/16 897, WO-A 01/02813, WO-A 01/02816, WO-A 02/099363, WO-A 03/048693. Especially, in U.S. Pat. Nos. 6,634,241, 6,487,917, 6,354,154, 6,343,517, 6,327,915, vibration-type measurement pickups, especially Coriolis mass flow pickups, are shown, which, in each case, include:                at least one measuring tube for the conveying of a fluid, which measuring tube has an inlet end and an outlet end and vibrates at least at times,        wherein the measuring tube, for enabling the fluid to flow through the measuring tube, communicates, via a first tube segment opening into the inlet end and via a second tube segment opening into the outlet end, with a pipeline connected therewith, and        wherein the measuring tube executes, during operation, mechanical oscillations about an imaginary oscillation axis connecting the two tube segments; and        a support element for the oscillatable holding of the measuring tube,        having a first end piece containing a passageway for the securement of the first tube segment and        having a second end piece containing a passageway for the securement of the second tube segment;        wherein each of the two tube segments extends through its respective one of the passageways and each of the two passageways has an inner diameter, which is greater than an outer diameter of its associated tube segment, so that an intermediate space is formed between each of the associated tube segments and end pieces.        
Additionally, measurement pickups of the described type include an exciter mechanism electrically connected with a corresponding measuring device electronics and serving for the driving of the at least one measuring tube. The exciter mechanism includes an oscillation exciter, especially an electrodynamic, or electromechanical, oscillation exciter, mechanically acting on the measuring tube. Such measurement pickups also include a sensor arrangement for delivering oscillation measurement signals. The sensor arrangement includes at least two sensor elements spaced from one another for reacting to vibrations of the measuring tube. During operation, the exciter mechanism is so actuated in suitable manner by the measuring device electronics by means of corresponding exciter signals, that the measuring tube executes, at least temporarily, vibrations, especially bending oscillations. For the sake of completeness, it is noted here that the illustrated support elements are usually completed to form a measurement pickup housing, which houses the at least one measuring tube, together with the oscillation exciters and sensors arranged thereon, as well as possible other components of the measurement pickup.
In principle, such measurement pickups, or transducers, come with two types of tube geometries, namely, on the one hand, straight measuring tubes and, on the other hand, bent measuring tubes, among which the U-shaped, or U-like, tubes are preferred tube shapes. Especially in the case of Coriolis pickups measuring mass flow, for reasons of symmetry, both types of tube geometries are most often used in the form of two measuring tubes extending in two parallel planes parallel to one another and, most often, also both containing fluid flowing through, naturally, also in parallel. For the one of the two variants having two parallel, straight tubes, reference can be made to, for example, U.S. Pat. Nos. 4,768,384, 4,793,191 and 5,610,342, and, for the other variant having two parallel, especially identically formed, U-shaped tubes, reference can be made e.g. to U.S. Pat. No. 4,127,028. Besides these types of mass flow pickups of double measuring tube arrangement working on the Coriolis principle, another type of measurement pickup has long been established in the market, namely those with a single straight or bent measuring tube. Measurement pickups of this type are described e.g. in U.S. Pat. Nos. 4,524,610, 4,823,614, 5,253,533, 6,006,609 or WO-A 02/099363.
For the case in which the measurement pickup being used involves one with a single, straight, measuring tube, the measurement pickup further includes a counter-oscillator affixed to the measuring tube and suspended, especially oscillatably, in the measurement pickup housing. The counter-oscillator serves, apart from holding the oscillation exciter and the sensor elements, for oscillatory decoupling of the vibrating measuring tube from the connected pipeline. This compensation cylinder can, in such case, be embodied e.g. as a tubular compensation cylinder, or box-shaped support frame, arranged coaxially with the measuring tube. To the referenced ensemble of features of the individual, above-described measurement pickups can also be added that a straight measuring tube, or the straight measuring tubes, as the case may be, is/are preferably made of pure titanium, a titanium alloy of high titanium content, pure zirconium or a zirconium alloy of high zirconium content, since, compared to measuring tubes of stainless steel, which is, per se, possible for straight measuring tubes, shorter installed lengths result, while a bent measuring tube, or bent measuring tubes, as the case may be, is/are preferably made of stainless steel, although titanium or zirconium, or their alloys, are also possible, in such case, as material of the measuring tubes.
In the case of inline measuring devices of the described kind, which are applied as Coriolis mass flow meters, their measuring device electronics determine, in operation, among other things, a phase difference between the two oscillation measurement signals delivered from the sensor elements and issue at their outputs a measurement signal derived therefrom, which presents a measured value corresponding with the behavior, over time, of the mass flow rate. If, as is usual for such inline measuring devices, also the density of the medium is to be measured, then the measuring device electronics determines additionally on the basis of the oscillation measurement signals an instantaneous oscillation frequency of the measuring tube. Moreover, also, for example, the viscosity of the medium can be measured on the basis of the power, especially a corresponding exciter current, for the exciter mechanism needed to maintain the oscillations of the measuring tube.
Besides the possibility of simultaneous measurement of a plurality of such process variables, especially mass flow, density and/or viscosity, by means of one and the same measuring device, there is another, significant advantage of inline measuring pickups of vibration-type, that they exhibit, among other things, a very high accuracy of measurement coupled with relatively little susceptibility to disturbances. Beyond this, such measuring devices can be used for practically any flowable medium and practically in any area of application in measurement and automation technologies.
In the manufacture of such measurement pickups of vibration-type, as already discussed in detail in U.S. Pat. Nos. 5,610,342, 6,047,457, 6,168,069, 6,598,281, 6,634,241 or also WO-A, the securement of the measuring tube within the support element, be it by welding, brazing, soldering and/or pressing, can be a special problem, especially with regard to the stability of the zero point and/or the availability of the measurement pickup. Additionally, as perceivable from U.S. Pat. Nos. 6,047,457, 6,168,069, 6,598,281, 6,634,241 or 6,523,421, considerable problems can also arise in the securement of the measuring tube inside of the support element, when measuring tube and support element are of different materials, for example titanium and high-grade steel, especially high-grade stainless steel.
As, furthermore, explained in, among others, also in U.S. Pat. Nos. 5,610,342, 6,047,457 or WO-A 03/048693, a suitable solution of the problem can be realized by affixing the measuring tube terminally in the support element by force- and/or interlocking-fit, with this force and/or interlocking fit being brought about by means of cold forming of the end pieces and/or of the tube segments. Studies of measurement pickups manufactured in this way have, however, shown, that the usually different expansion characteristics of the above-mentioned end pieces and the tube segments of the measuring tube held therein can lead to the fact that the clamping forces exerted by the end pieces on the measuring tube fall, in the case of temperature fluctuations, especially in the case of possible temperature shocks, such as can arise e.g. in the case of regularly performed cleaning operations with extremely hot washing liquids, below a critical value. This can, in turn, mean that the end piece and measuring tube lose, because of thermally-related expansions, the mechanical contact brought about by the rolling and, consequently, the support element becomes rotatable about the above-mentioned oscillation axis, relative to the measuring tube. After that, especially in the case of measurement pickups executing, during operation, also torsional oscillations about the oscillation axis, with a slipping of the support being an event that is no longer, with certainty, out of the question, a replacement of the entire measuring device becomes practically unavoidable. Comparable effects have been discussed in this connection also in WO-A 03/048693 and U.S. Pat. No. 6,598,281.